The Serial Link Module (SLM) controls serial I/O data transfer to and from remote I/O. It is the intermediary between the 620 CPM and the SIOMs. The SLM collects input data from the SIOMs. The CPM reads this data during its input status scan. Then, according to its program instructions, the CPM issues output data to the SLM, which sends it to the SIOMs. The SLM has its own microprocessor and data table, and operates asynchronously from the 620 CPM.

The serial I/O configuration allows the control system to operate over long distances, so that serial I/O racks can be mounted close to the machine or process rather than at the logic controller. Such remote I/O operation allows the system to be set up in either a star or a multidrop configuration.

The key interface modules in a serial configuration are the Serial Link Module (SLM) (621-9939C) and the
Serial I/O Module (SIOM) (621-9938RC or 621-994OC). Each SLM acts as a master to a maximum of two serial channels, transmitting PUSH data and other output data to each SIOM. In tum, the SIOMs communicate PULL data and other input data back to the SLM.

In a serial configuration, one SIOM must reside in the rightmost slot of each serial I/O rack, and at least
one SLM must reside in the processor rack.